The invention relates to a method for ascertaining a distance of an object from a motor vehicle using a PMD sensor and a PMD sensor.
PMD sensors (Photonic Mixing Device-Sensor) are well-known in the art as optical sensors, whose functional principle is based on the time-of-flight-principle (time of flight). A measuring signal emitted from a signal source is reflected at an object and is received again by a signal detector. Hereby, light-measuring signals, modulated by a modulation frequency, are usually used, wherein a phase shift is determined by comparing the emitted and the received measuring signal. From this phase shift, distances to the object can be calculated, albeit with ambiguity, because since the phase shift is considered, each distance that is modified by a multiple of the modulation wave length associated with the modulation frequency results in the same phase shift.
The use of low modulation frequencies is usually not desired because even though long wavelengths are present, the measurement and thus the duration of a measurement cycle is significantly prolonged, so that distance measurements are not available at the desired frequency. The use of high frequencies, however, offers many advantages because the overall efficiency of the PMD sensor increases with increasing frequency. Especially the local resolution and range of the measuring signal can be enhanced. However, higher frequencies pose the problem that a detection range, which begins at the motor vehicle, and within which an unambiguous determination of a distance based on the phase shift is possible, is correspondingly shorter. A further problem is that, at usual illumination intensities that are adjusted to such a detection range or definiteness range, highly reflective objects, e.g. so-called retro-reflectors, may still generate a signal at greater distances which, when only considering the closest possible distance, may generate an erroneous signal due to the phase shift.
To solve this problem in applications using great detection ranges it was proposed to expand the detection range by combining multiple frequencies. Hereby, the measuring signal is modulated within a measurement cycle by several different modulation frequencies, for example three modulation frequencies, and phase shifts are correspondingly determined for each modulation frequency. From these phase shifts multiple possible distances result as described above. Only distances that were determined for all phase shifts are possible solutions so that, depending on the selection of the different modulation frequencies, the detection range within which an unambiguous identification of a distance is possible can be expanded.
However, this solution has the disadvantage that due to the number of modulation frequencies, the time until a reliable (unambiguous) identification of a distance to an object is determined by the required exposure times. This significantly prolongs the measurement cycle. In addition this solution is limited by the selection of the possible frequencies, particularly by the upper maximum frequency. This limitation prevents the optimal combination of amount of light and modulation frequency.